A boosted engine may offer greater fuel efficiency and lower emissions than a naturally aspirated engine of similar power. During transient conditions, however, the power, fuel efficiency, and emissions-control performance of a boosted engine may suffer. Such transient conditions may include rapidly increasing or decreasing engine load, engine speed, or mass air flow. For example, when the engine load increases rapidly, a turbocharger compressor may require increased torque to deliver an increased air flow. Such torque may not be available if the turbine that drives the compressor is not fully spun up. As a result, an undesirable power lag may occur before the intake air flow builds to the required level.
It has been recognized previously that a turbocharged engine system may be adapted to provide “blow-through” air wherein boosted intake air is driven from the intake manifold, downstream of the compressor, through the engine cylinder(s) and into the exhaust manifold, upstream of the turbine. For example, a variable cam timing (VCT) system may be temporarily adjusted to provide high valve overlap. During the positive valve overlap, the boosted air is inducted through the cylinders into the turbine to temporarily provide extra mass flow and enthalpy in the exhaust. The extra turbine energy enables the turbine to spin-up faster, thereby reducing turbo lag.
However, the inventors herein have identified potential issues with such an approach. As one example, in order to provide the blow-through, the engine has to be in a positive pumping regime (that is, under boosted engine operation), else turbocharger performance may be degraded. Further, during blow-through, the engine may be operated with high levels of spark retard in order to provide additional energy to the exhaust for increasing turbine speed and boost. However, operating the engine with high levels of spark retard may cause combustion to occur later than an optimal timing required for immediate torque output.
Thus, at least some of the above issues may be addressed by a method for a turbocharged engine comprising: supplying compressed air through a throttle to an engine from a compressor driven by a turbine coupled to an exhaust of the engine; and during tip-in of the throttle, reducing turbo-lag by delivering ambient air to the turbine during a first mode of operation and during a second mode of operation providing blow-through of a portion of the compressed air through the engine, without combustion, to the turbine.
As an example, in response to a tip-in, a secondary air pump may be utilized to deliver secondary air in to an exhaust manifold upstream of a turbine. At the same time, enrichment may be provided to generate high levels of engine out carbon monoxide (CO), hydrogen (H2), and hydrocarbons to react with the secondary air in the exhaust. As a result, mass and enthalpy of the exhaust may be increased, which may be utilized to increase a speed of the turbine to a desired speed. Further, due to increase in turbine speed, a boost pressure may be increased. Upon attaining a threshold boost pressure at which sufficient boost may be available for blow-through, engine operation may switch to providing blow-through. That is, secondary air injection may be stopped and blow-through may be utilized to provide extra air in the exhaust.
In some examples, upon reaching the threshold boost pressure, a secondary air injection amount may be decreased and simultaneously, blow-through air amount may be increased until the secondary air injection amount decreases below a threshold amount, after which engine may be operated with blow-through only until the desired turbine speed is achieved.
By providing secondary air injection early during the tip-in, sufficient boost may be generated, which may be utilized for blow-through. As a result, turbocharger performance may be improved. Further, utilizing secondary air injection during the initial part of the tip-in may allow higher trapped mass in the cylinder (since the extra air in the exhaust required for extra exhaust energy is provided by the secondary air pump instead of the turbocharger). As a result, initial torque output may be improved. Additionally, by utilizing secondary air injection during the early part of tip-in, valve timings may be adjusted to increase initial torque output. Still further, by switching engine operation to additionally or alternatively provide blow-through after sufficient boost is attained, the time required for the turbine to reach a desired speed may be reduced, thereby reducing turbo lag. In this way, secondary air injection and blow-through may be coordinated during tip-in to expedite turbine spin-up and improve initial torque output.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.